Perineal disease is a frequent complication of Crohn's disease. Approaches to therapy have included, surgery and various medications, which may require time to work or can cause significant toxicity. Several papers have described the beneficial results of hyperbaric oxygen therapy (HBO) in patients with severe or refractory perineal disease. The role of hyperbaric therapy in larger groups or less severely affected patients has not been studied. Additionally, the minimum number of treatments required for initial or complete healing of perineal disease in this population has not been described. Patients with documented perineal Crohn's disease who meet the eligibility criteria will be randomized to treatment with HBO or observation, then crossed over to the other group after 6 months. Crohn's disease activity will be measured biweekly using the Harvey-Bradshaw index, and perineal disease monitored biweekly using the Cardiff Classification. Patients will be enrolled on an intention-to-treat basis. A maximum of twenty sessions of HBO therapy will be given to each patient. The endpoint is complete resolution of perineal disease. We will monitor side effects of HBO therapy including inner ear barotrauma, pulmonary complications, development of visual difficulties and claustrophobia, among others.